Active sonar systems create an acoustic impulse, and then listen for reflections of the impulse to detect the location of objects under the water such as hostile submarines and submerged mines, to detect the features of undersea terrain and to detect the presence of undersea gas and oil deposits. A variety of active sonar systems generate an acoustic impulse, such as a ping, from an electronic acoustic source located in the same place as the receiver. In other systems, however, the position of the acoustic source is spatially separated from the receiver. For example, an impulse acoustic source may be dropped from a ship or aircraft to a predetermined depth and activated. Such impulse acoustic sources are generally expendable and may include, for example, explosive charges, pneumatic devices, or electronic (sparker) signal generators. Small explosive charges are frequently used as impulse acoustic sources in airborne anti-submarine warfare (ASW) sonar systems because they are light in weight, compact, and provide good depth of penetration.
In cases where the detonation depth is extremely deep, explosive impulse acoustic sources have been modeled as free falling objects in a column of water and detonated with a pressure actuated fuse. The detonation depth tolerance of free falling explosive impulse acoustic sources is typically 10% of the depth setting. In practice, detonation depths are even more variable since older explosives can detonate at depths that are significantly outside of the 10% tolerance range. Although free falling explosive impulse acoustic sources have many advantages, the lack of accuracy in determining the depth of the explosion causes sonar measurements to be less accurate since source detonation depth is needed to achieve an acceptable tolerance on measured propagation loss estimates. Thus an accurate estimate of the activation depth for an impulse acoustic source is needed. Embodiments according to the present invention are directed to solving the foregoing problems.